1. Field of the Invention
The present invention relates to a packet transmission control apparatus and a packet transmission control method which perform transmission control of packets to a plurality of mobile stations.
The present invention relates particularly to a packet transmission control apparatus and a packet transmission control method which perform transmission control (scheduling) of downlink packets in a mobile communication system.
2. Description of the Related Art
In a downlink of a mobile communication system, one physical channel can be shared among mobile stations which belong to the radio base station. Hereinafter, the physical channel used in such a case is called a “downlink shared channel”.
In this downlink shared channel, the radio base station controls the transmission order of packets to the plurality of mobile stations with which the radio base station communicates, in accordance with an instantaneous radio quality between the radio base station and each mobile station, so as to improve throughput that the radio base station can provide, in other words, a system capacity.
This control of packet transmission order by the radio bass station is called “scheduling”. It is known that, by applying the scheduling to packet transmission, channel capacity increases, or communication quality improves.
Generally, it is considered that the conventional scheduling targets on packets in which requirements for transmission delays are not so strict.
Incidentally, with regard to standardization of the third generation mobile communication system, so-called IMT-2000, there are “3GPP/3GPP2 (Third-Generation Partnership Project/Third-Generation Partnership Project 2)”. Standard specifications have been developed as “W-CDMA system” in the 3GPP, and standard specifications have been developed as “cdma2000 system” in the 3GPP2.
In the 3GPP, “HSDPA (High Speed Downlink Packet Access)”, which is a high-speed packet transmission system in the downlink direction, has been standardized based upon a prospect that high-speed and high-capacity traffic will increase especially in the downlink due to downloading from databases and websites and the like, as the Internet has rapidly expanded in recent years.
Moreover, in the 3GPP2, “1x-EV DO”, which is a transmission system only for high-speed data in the downlink direction, has been standardized from the same viewpoint as above. In the “1x-EV DO” of the cdma2000 system, “DO” means “Date Only”.
For example, in the HSDPA, a scheme for controlling a modulation scheme and a coding rate of respective radio channels in accordance with the radio condition between each mobile station and a radio base station (this scheme is called, for example, AMCS (Adaptive Modulation and Coding Scheme) in the HSPDA), and the scheduling which is operated in a cycle of few milliseconds, are used in a combination. Thus, it is possible to improve throughput for individual mobile stations as well as throughput of the entire system.
“Round Robin Scheduler” is widely known as a scheduling algorithm in a radio base station. The “Round Robin Scheduler” controls the transmission order of packets waiting for transmission, by assigning the downlink shared channel sequentially to mobile stations (for example, mobile stations #1 to #2 to #3 . . . ) which belong to the radio base station.
Moreover, “Proportional Fairness Scheduler” and “Max C/I (Maximum C/I) Scheduler” are known as scheduling algorithms in a radio base station. The “Proportional Fairness Scheduler” and the “Max C/I Scheduler” control the transmission order of packets waiting for transmission based upon the radio condition between the radio base station and each mobile station and the average transmission rate of packets to each mobile station.
The “Proportional Fairness Scheduling” is a scheduling algorithm which assigns a transmission queue and also supports fairness amongst the mobile stations, in accordance with instantaneous changes in downlink conditions of the individual mobile stations.
Hereinbelow, the “Proportional Fairness Scheduling” is briefly described with reference to FIG. 1. FIG. 1 is a flowchart showing the operation of the “Proportional Fairness Scheduler” mounted on a radio base station.
In the “Proportional Fairness Scheduling”, a value of an evaluation function of each mobile station which belongs to the radio base station is calculated based upon the measured radio condition between each mobile station and the radio base station and the measured average transmission rate of packets to each mobile station, and thereafter, a transmission queue is assigned to a mobile station maximizing the value of the evaluation function.
As shown in FIG. 1, in step S1001, the radio base station sets initial values as follows:    n=1 (n: a subscript of a mobile station)    Cmax=0 (Cmax: a maximum value of an evaluation function Cn)    nmax=0 (nmax: a subscript of the mobile station maximizing the value Cn of the evaluation function)
In step S1002, the radio base station measures elements required in calculating the value Cn of the evaluation function, specifically, an instantaneous radio condition Rn between the radio base station and each mobile station #n, and an average transmission rate     Rnof packets to each mobile station #n.
In step S1003, the radio base station calculates the value Cn of the evaluation function, by using the values measured in the step S1002, according to the following equation.
  Cn  =      Rn          Rn      _      
In step S1004, the radio base station determines whether the value Cn of the evaluation function calculated in the step S1003 exceeds a maximum value Cmax of the evaluation function.
Here, Cmax=0. Therefore, the determination in the step S1004 is YES, and in step S1005, the radio base station sets the value Cn of the evaluation function calculated in the step S1003 at the maximum value Cmax of the evaluation function, and also sets “1” at “nmax”.
Thereafter, in step S1006, the radio base station increments “n” by “+1”, and determines whether “n” exceeds “N (the number of mobile stations communicating with the radio base station)” in step S1007.
Where “n” does not exceed “N”, the operation repeats the steps from S1002 to S1006, thus obtaining N values of the evaluation function sequentially.
In step S1008, the radio base station selects a mobile station #nmax maximizing the value Cn of the evaluation function, and assigns a transmission queue to the mobile station #nmax.
A radio base station, on which the “Proportional Fairness Scheduler” is mounted, assigns a transmission queue to each mobile station #n while downlink quality (radio condition) is relatively good. Therefore, higher throughput can be expected in comparison with a radio base station with the “Round Robin Scheduler” mounted thereon.
Furthermore, in the “Proportional Fairness Scheduling”, the radio condition between the radio base station and each mobile station is divided by the average transmission rate of packets to each mobile station, thus lowering the value of the evaluation function of a mobile station with a high average transmission rate. Thus, the transmission queue can be assigned with high fairness in terms of time, compared to the “MAX C/I Scheduling” as describe later.
Meanwhile, the “MAX C/I Scheduling” is a scheduling algorithm which assigns a transmission queue to a mobile station with the best downlink quality (radio condition) amongst mobile stations which belong to the radio base station.
In other words, in the “MAX C/I Scheduling”, the same processing as that of the “Proportional Fairness Scheduler” is carried out except that the evaluation function Cn in the processing of the “Proportional Fairness Scheduler” is set so that “Cn=Rn”.
In the case of the “MAX C/I Scheduler”, a transmission queue is assigned to a mobile station with good downlink quality at the beginning of a scheduling cycle.
Normally, a transmission rate of respective packets becomes higher depending on the quality of a link. Therefore, in the “MAX C/I Scheduling”, a transmission opportunity is provided to a mobile station with the highest transmission rate.
However, in the “MAX C/I Scheduler”, few transmission opportunities are provided to a mobile station with poor average downlink quality, such as a mobile station located remotely from the radio base station, This causes a problem in that throughput obtained at each mobile station is extremely different from one another.
In other words, the “MAX C/I Scheduler” causes a situation where a mobile station located in the vicinity of the radio base station can obtain extremely good throughput, whereas the remaining mobile stations obtain low throughput.
As described so far, in the conventional mobile communication system, the scheduling has been carried out by setting the above-mentioned evaluation function in consideration of the type of service provided and priority related to the type of service, based upon the “Round Robin Scheduler”, the “Proportional Fairness Scheduler”, or the “MAX C/I Scheduler”.
Generally, a scheduling algorism is evaluated based on two criteria which are “fairness in terms of time” and “high cell throughput by user diversity gain”.
Here, among the above three types of schedulers, the “Proportional Fairness Scheduler”, which can realize “fairness in terms of time” and “high cell throughput by user diversity gain”, is considered.
The “Proportional Fairness Scheduler” uses an evaluation function of
  Cn  =      Rn          Rn      _      in which its numerator is an “instantaneous transmission rate of packets to the mobile station #n (instantaneous radio condition with the mobile station #n)”, and its denominator is an “average transmission rate of packets to the mobile station #in”. Thus, the “Proportional Fairness Scheduler” operates so that the probability of scheduling packets to the above mobile station, whose instantaneous transmission rate is larger than the above average transmission rate, becomes higher.
Packets are scheduled to a mobile station which has high instantaneous radio quality, even when the average transmission rate of the mobile station is large or small. Therefore, the “Proportional Fairness scheduler” can realize both “fairness in terms of time” and “high cell throughput by user diversity gain”.
However, the operation of the conventional “Proportional Fairness Scheduler” is based on the assumption that the range of changes in radio quality is the same in all mobile stations.
In the case where the radio quality of a first mobile station changes within a large range and the radio quality of a second mobile station changes within a small range, the probability of scheduling packets to the second mobile station becomes low since the value of the evaluation function of the second mobile station is constantly small. Thus, there has been a problem in that fairness in terms of time cannot always be provided by the conventional “Proportional Fairness Scheduler”.
In other words, the conventional “Proportional Fairness Scheduler” is easy to schedule packets to a mobile station whose radio quality changes are large, but is hard to schedule packets to a mobile station whose radio quality changes are small. Therefore, there has been a problem in that fairness in terms of time cannot always be provided by the conventional “Proportional Fairness Scheduler”.
For example, in the case where there are two mobile stations whose radio qualities are equal, the changes in the radio quality of the first mobile station is large and the change in the radio quality of the second mobile station is small, the conventional “Proportional Fairness Scheduler” tends to schedule packets to the first mobile station, comparing to the second mobile station. As a result, the conventional “Proportional Fairness Scheduler” cannot realize fairness in terms of time.
The followings are examples of considerable cases where the radio quality of the mobile station does not change:                the case where the mobile station is in a stationary state;        the case where the mobile station is in a multipass environment (the case where a pass diversity exists):        the case where the mobile station is in use of a receive diversity process, an equalizer or the like.        
It is explained, by referring to FIG. 2, that the radio quality changes of a mobile station becomes small when the mobile station is in use of a receive diversity process. As shown in FIG. 2, the radio qualities which change without correlation are combined by the mobile station which uses a receive diversity process and therefore the radio quality changes are reduced.